1. Field of the invention
The invention relates to a hydraulic control apparatus of an automatic transmission installed in a motor vehicle and, more particularly, to a hydraulic control apparatus for controlling lag time that elapses between a shift decision and the actual start of the shift.
2. Description of the Related Art
A typical automatic transmission automatically makes a shift decision (determination) in accordance with a predetermined speed shift map based on the vehicle speed and the throttle opening. Responsive to the shift decision, the automatic transmission supplies a predetermined oil pressure to or drains oil pressure from hydraulic servos for operating clutches or brakes (frictional engagement elements), thus changing the torque transmission path through a multi-speed shift gear mechanism to accomplish the shift.
The shift map is prepared so that the vehicle speed at which a downshift is to be performed decreases as the throttle opening decreases. However, if the vehicle speed rapidly decreases as in full braking, the vehicle speed may fall to a speed equal to or less than the vehicle creep speed of the current gear ratio (pre-shift speed) during an oil pressure response lag time between a shift decision (shift instruction) and the actual shift. If a downshift is effected during coasting, i.e. the drive state is changed from non-driven to driven, a shift shock occurs.
A conventional hydraulic control apparatus of an automatic transmission is disclosed, for example, in Japanese patent Laid-Open application ("Kokai") No. HEI 4-78370, and includes vehicle speed changing rate calculating means for calculating the vehicle speed changing rate from vehicle speed information and, if the throttle opening is equal to or less than a predetermined value and the vehicle speed changing rate is equal to or greater than a predetermined value, controls the speed shift by correcting the vehicle speed value predetermined for a downshift in a speed shift map, i.e. that predetermined vehicle speed is increased by a predetermined amount.
This conventional apparatus can prevent the vehicle speed during a downshift from falling to or below the creep vehicle speed of the current gear stage (gear ratio), even though oil pressure response lag occurs between the time of a shift decision and the time of the actual shift (as in full braking), thus reducing shift shock.
The above-described conventional apparatus does not correct a shift point in the speed shift map unless the throttle opening is equal to or less than the predetermined value and the vehicle speed changing rate (deceleration rate) is equal to or greater than the predetermined value. However, even when the deceleration rate is equal to or less than the predetermined value, a shift shock can be caused by changeover of the drive state during a shift.
The conventional apparatus also has a drawback with upshifts. For example, if an upshift is performed during acceleration from a high vehicle speed, the engine speed may rise to or above an allowable speed limit during the shift, thus increasing wear of the engine.